1. Field of the Invention
The invention relates to a malfunction diagnostic apparatus and a malfunction diagnostic method for a combustion improvement device. More specifically, the invention relates to a malfunction diagnostic apparatus and a malfunction diagnostic method for a combustion improvement device, which determine whether a malfunction occurs in a combustion improvement device for each of cylinders, with low cost, and with high determination performance.
2. Description of the Related Art
Conventionally, an internal combustion engine in which a combustion improvement device is provided is available. The combustion improvement device, disposed in an intake passage connected to a combustion chamber, improves a combustion state. As an example of the combustion improvement device, an airflow control valve is generally available. The airflow control valve generates a tumble flow and a swirl flow. When a malfunction occurs in the combustion improvement device, combustion is not performed in an intended manner. Accordingly, the combustion state is deteriorated, and as a result, the combustion state varies among cylinders, or emissions are increased. Thus, it is important to determine whether a malfunction occurs in the combustion improvement device with high determination performance, to change a control to another control for dealing with a malfunction, or quickly repair a malfunction when a malfunction occurs.
With regard to determination as to whether a malfunction occurs in the combustion improvement device, for example, Japanese Patent Application Publication No. 10-121991 (JP-A-10-121991) describes a technology in which it is determined whether a malfunction occurs in an intake control valve using an air-fuel ratio. Japanese Patent Application Publication No. 07-83101 (JP-A-07-83101) describes a technology in which it is determined whether a malfunction occurs in an SVC (Swirl Control Valve) by comparing a normal combustion period with an actual combustion period using a combustion pressure sensor. Japanese Patent Application Publication No. 09-209810 (JP-A-09-209810) describes a technology in which it is determined whether a malfunction occurs in a TCV (Tumble Control Valve) based on the difference in the pressure upstream of the TCV between when the TCV is open and when the TCV is closed, using a pressure sensor. Each of the intake control valve, the SCV, and the TCV is equivalent to the combustion improvement device. With regard to determination on the combustion state, for example, Japanese Patent Application Publication No. 2004-92603 (JP-A-2004-92603) describes a technology in which the combustion state in each cylinder is estimated using a crank angular acceleration. Japanese Patent Application Publication No. 2005-61371 (JP-A-2005-61371) describes a technology in which a cylinder, in which the combustion state is deteriorated, is determined using a crank angular acceleration, and the combustion state is recovered by correcting an ignition timing and the like.
For example, in the technology using the air-fuel ratio described in the publication No. 10-121991, it is not possible to determine the cylinder corresponding to the combustion improvement device in which a malfunction occurs. In this case, when it is determined that a malfunction occurs, it is not possible to restrict the operating condition in each cylinder. Therefore, for example, when a control is changed to another control for dealing with the malfunction, the operation in the normal cylinders may be spoiled. Examples of the malfunction conditions of the combustion improvement devices are as follows. All the airflow control valves may be kept in the open position or closed position, and therefore all the airflow control valves may malfunction. At least one airflow control valve may be deformed or broken, or may fall, and therefore, the combustion state may be deteriorated in the cylinder corresponding to the at least one airflow control valve. Also, the combustion state may be deteriorated in a specific cylinder due to the torsion of a valve shaft that axially supports the airflow control valves. The airflow control valve may be deformed or broken, or fall, when the airflow control valve is made of resin, and a back fire occurs.
In the technologies described in the publication No. 07-83101 and the publication No. 09-209810, it is possible to determine whether a malfunction occurs in the combustion improvement device for each cylinder. However, in the technologies described in the publication No. 07-83101 and the publication No. 09-209810, a cylinder pressure sensor or an intake passage pressure sensor needs to be newly provided to determine whether a malfunction occurs, in the case where such a sensor is not provided. Therefore, the cost of the entire configuration may be increased. Also, in the technologies, the determination performance is not taken into account. Therefore, when a relatively minor malfunction occurs, for example, a malfunction occurs due to the deformation of the airflow control valve, it may not be possible to determine that the minor malfunction occurs. In the technologies described in the publication No. 2004-92603 and the publication No. 2005-61371, it is not possible to determine whether the combustion state is deteriorated due to the malfunction of the combustion improvement device, or due to inhalation of air into an intake system, or due to the malfunction of a fuel injection system. Therefore, in the technologies described in the publication No. 2004-92603 and the publication No. 2005-61371, a control may not be switched to an appropriate control for dealing with a malfunction, according to the cause of the malfunction, or it may take much time to determine the cause of the malfunction to repair the malfunction, and thus, the malfunction may not be quickly repaired.